In the hot rolling of metal products, in particular flat products such as strip or sheet, it is known that it is necessary to provide, for example at exit from the heating and/or maintenance systems, devices suitable to remove the surface scale generated due to the exposure to the atmosphere of the metal product at high temperature.
Known scale-removal systems are substantially divided into mechanical, chemical or chemical-mechanical methods depending on how they are achieved.
Among the mechanical methods, it is known for example to strike the surface of the material to be cleaned, in movement, with solid particles having a desired hardness and shape, which are thrown at high speed against the surface to be cleaned.
Depending on the desired result and the type of material to be treated, the solid particles consist of cast iron or steel balls; alternatively, steel brushes rotating at high speed can be used.
In rolling lines on the contrary it is known to use generally a method to remove the scale using jets of water at high pressure, which are directed with a suitable inclination onto the moving metal product.
In this method, the moving metal product is hit, both below and above, by jets of water at a pressure between around 10-18 MPa.
Known devices that carry out this method normally use nozzles—fixed, rotating and/or translating—which emit the jet of water toward the surface of the metal product, operating on the physical principle of heat shock, to which the scale is subjected on contact with the water, which causes it to detach from the surface of the product and to be consequently removed.
However, it has been found that this method, which uses jets of pressurized water emitted from nozzles, is not suitable to remove the scale when the thickness of the metal product goes below a certain value (for example ≦4-5 mm), since the jets of water emitted by the nozzles cause an excessive cooling of the product, which therefore comes out from the scale-removal device too cold, making the subsequent rolling pressures thereof increase excessively.
To prevent the product cooling, the pressure and hence the flow rate of the scale-removal water should be reduced, but if scale remains on the surface of the product, in the subsequent rolling step the scale becomes deeply impressed in said surface, and therefore the quality of the final product obtained deteriorates.
This entails a considerable economic damage, since the resultant final product is lower quality and hence a lower sales price is given to it.
If this problem is not particularly important in continuous train rolling mills, since the scale is removed at entrance to the first stand in the train when the thickness of the strip is still sufficiently high (for example ≧10-12 mm), and the scale does not have time to reform in the short inter-stand segments, on the contrary it is very important in Steckel reversing rolling mills.
In this type of stands a rolled product passes several times in one direction and the other through one or two reversing stands, reducing in thickness at every pass, and winding inside a winding/unwinding furnace after each pass.
To prepare the strip for rolling, between the winding/unwinding furnace and the reversing stand a scale-removal device may by provided.
As we said before, however, when the thickness of the strip goes below a certain value, for example less than 8 mm, or even more if less than 5 mm, the scale-removal device must be de-activated, since the action of the jets of water emitted by the nozzles would cause an excessive cooling of the surface of the strip such as to render the subsequent rolling pressure excessive.
This problem has until now limited the potential of reversing mills, in practice preventing the use of scale-removal devices for very thin products, for example 1.5-3 mm.
U.S. Pat. No. 2,921,748 and SU-A1-624.676 describe scale-removal devices comprising nozzles associated with a box-like structure having a fissure for the water to exit which extends for the width of the metal product.
The purpose of the invention is therefore to obtain a device, and the corresponding method, to totally remove the scale, and to contain the loss of temperature of the surface of the strip in the rolling step, also on products with a very thin thickness (for example ≦5 mm), so as not to create problems in the subsequent rolling steps.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.